The present invention relates to a technique in the geological and resource fields for measuring porosity of a rock which forms a stratum, and more particularly, to a method for measuring porosity of a rock by using an SEM image.
To qualitatively and quantitatively determine the distribution of pores in a rock is a very important technique. For example, when the porosity in an oil gas-storage reservoir is measured, an amount of oil and gas resources may be calculated, and when information about porosity is used to calculate a transmittance, how much and how the oil and gas resources may be recovered may also be determined.
Accordingly, regarding techniques for measuring porosity of rocks forming a stratum, a variety of methods have been developed, existing methods may be continuously improved by succeeding research, or novel methods may also be developed.
In particular, since resources in traditional reservoirs such as sandstone are exhausted, resource extraction is performed in a non-traditional reservoir such as dense strata or shale, which has very small pores and a much more complicated pore structure. Accordingly, the demand for a technique of more efficiently and accurately measuring a fine-scale porosity in such rocks has come to the fore.
Meanwhile, the present inventors published a paper in which pros and cons of all sorts of techniques widely used for analyzing pores in rocks, and the description herein will be provided with reference to the corresponding paper.    Related paper: Jae Hwa Jin, Junho Kim, Jeong-Yil Lee, and Young Min Oh, 2016, Correlative multiple porosimetries for reservoir sandstones with adoption of a new reference-sample-guided computed-tomographic method, Scientific Report 6 (30250): 1-10.
Referring to the related paper, techniques of analyzing pores in a rock which have been used since the development thereof may be classified into three categories, for example: 1) a technique of extracting all substances distributed in the pores in a rock, that is, pore-filling substances such as oil and water and measuring the amount thereof, thereby additionally securing pore information; 2) a technique of forcibly inserting fluid substances in the pores and measuring porosity on the basis of the inserted amount; and 3) a technique of imaging the rock using an electronic apparatus, and then separating and identifying the pores from a medium by using a graylevel difference on the image of the medium and the pores from the captured image and thereby embodying a pore structure and measuring porosity of the pore structure; and the like.
As introduced in the related paper, the techniques of example 1) among the above-mentioned pore analyzing techniques include a Dean-Stark method, a retort method, and the like, the techniques of example 2) include a mercury intrusion porosimetry, a helium gas intrusion method, a gas adsorption method, and a ultra-pure water immersion method, and the like, and the techniques of example 3) which are more actively being studied recently include a method of imaging the medium and pores of a rock to be analyzed by using an electronic apparatus such as X-ray computed tomography (CT) or scanning electron microscopy (SEM), and then quantitatively calculating the characteristic values of the pores.
However, each of the above-mentioned pore measuring methods has a measuring limit. For example, in the case of a method such as that of example 1), substances filling the pores should be, of course, conserved as it is before a pore measuring experiment, samples need to be pulverized to extract the corresponding substances completely, and information about a pore structure is therefore difficult to obtain. In the case of example 2), a measuring limit is determined for each kind of injected substance for pore measurement. That is, in the case of the mercury intrusion porosimetry, a pore having a very small size of nanometer level is difficult to measure, and conversely, in the gas adsorption method, a large pore having a size level of micrometer or greater is difficult to measure. In addition, when a gas is used as in the helium gas intrusion method or the gas adsorption method, since a bulk volume data need to be borrowed from other methods, the method is difficult to be a completely independent measurement, and in the case of the ultra-pure water, the accuracy of the pore measurement is unsatisfactory.
Meanwhile, in the case of example 3), there is a merit in that not only a qualitative measurement of the pores but also the information about a pore structure may be obtained together because the medium and pores of a given sample are imaged as it is, but conversely, has a limitation in that pores having nanometer-level sizes are difficult to measure due to a resolution limit in the current technical level of CT technology.
When an SEM is used, a two-dimensional pore structure is obtained by processing the sample into a thin-section sample and observing the surface of the sample. Meanwhile, since a specific portion of the sample may be imaged to be magnified up to several thousands of thousand times, in particular, rocks of non-conventional reservoir in which pores having nanometer-level sizes occupy majority may be helpfully measured. The technique using an SEM may be the best in terms of accuracy. However, in the conventional SEM image analysis method, when pores and other rock media inside a rock to be analyzed are not clearly identified on an SEM image, the pore measurement becomes inaccurate. The reason for this is because media of the whole or a portion of the rock to be analyzed are formed of substances having electron densities not so higher than those of the pores and thereby does not show a clear graylevel difference on an SEM image compared to the pores. A representative example is the case in which the whole or a portion of the media in a rock is formed of low-density substances such as porous clay mineral aggregates or solid kerosene. Pores present in association with such substances have graylevels that are not so different from the surrounding medium substances, have sizes that are also very small in such a degree to reach a micrometer or nanometer level, and thus, have limitations of not being easily identified even on an SEM image.